In mathematics, a dual system, dual pair or a duality over a field ${\displaystyle \mathbb {K} }$ is a triple ${\displaystyle (X,Y,b)}$ consisting of two vector spaces, ${\displaystyle X}$ and ${\displaystyle Y}$, over ${\displaystyle \mathbb {K} }$ and a non-degenerate bilinear map ${\displaystyle b:X\times Y\to \mathbb {K} }$.

In mathematics, duality is the study of dual systems and is important in functional analysis. Duality plays crucial roles in quantum mechanics because it has extensive applications to the theory of Hilbert spaces.

A pairing or pair over a field ${\displaystyle \mathbb {K} }$ is a triple ${\displaystyle (X,Y,b),}$ which may also be denoted by ${\displaystyle b(X,Y),}$ consisting of two vector spaces ${\displaystyle X}$ and ${\displaystyle Y}$ over ${\displaystyle \mathbb {K} }$ and a bilinear map ${\displaystyle b:X\times Y\to \mathbb {K} }$ called the bilinear map associated with the pairing, or more simply called the pairing's map or its bilinear form. The examples here only describe when ${\displaystyle \mathbb {K} }$ is either the real numbers ${\displaystyle \mathbb {R} }$ or the complex numbers ${\displaystyle \mathbb {C} }$, but the mathematical theory is general.

For every ${\displaystyle x\in X}$, define $${\displaystyle {\begin{alignedat}{4}b(x,\,\cdot \,):\,&Y&&\to &&\,\mathbb {K} \\&y&&\mapsto &&\,b(x,y)\end{alignedat}}}$$ and for every ${\displaystyle y\in Y,}$ define $${\displaystyle {\begin{alignedat}{4}b(\,\cdot \,,y):\,&X&&\to &&\,\mathbb {K} \\&x&&\mapsto &&\,b(x,y).\end{alignedat}}}$$ Every ${\displaystyle b(x,\,\cdot \,)}$ is a linear functional on ${\displaystyle Y}$ and every ${\displaystyle b(\,\cdot \,,y)}$ is a linear functional on ${\displaystyle X}$. Therefore both $${\displaystyle b(X,\,\cdot \,):=\{b(x,\,\cdot \,):x\in X\}\qquad {\text{ and }}\qquad b(\,\cdot \,,Y):=\{b(\,\cdot \,,y):y\in Y\},}$$ form vector spaces of linear functionals.

It is common practice to write ${\displaystyle \langle x,y\rangle }$ instead of ${\displaystyle b(x,y)}$, in which in some cases the pairing may be denoted by ${\displaystyle \left\langle X,Y\right\rangle }$ rather than ${\displaystyle (X,Y,\langle \cdot ,\cdot \rangle )}$. However, this article will reserve the use of ${\displaystyle \langle \cdot ,\cdot \rangle }$ for the canonical evaluation map (defined below) so as to avoid confusion for readers not familiar with this subject.

A pairing ${\displaystyle (X,Y,b)}$ is called a dual system, a dual pair, or a duality over ${\displaystyle \mathbb {K} }$ if the bilinear form ${\displaystyle b}$ is non-degenerate, which means that it satisfies the following two separation axioms:

In this case ${\displaystyle b}$ is non-degenerate, and one can say that ${\displaystyle b}$ places ${\displaystyle X}$ and ${\displaystyle Y}$ in duality (or, redundantly but explicitly, in separated duality), and ${\displaystyle b}$ is called the duality pairing of the triple ${\displaystyle (X,Y,b)}$.

A subset ${\displaystyle S}$ of ${\displaystyle Y}$ is called total if for every ${\displaystyle x\in X}$, $${\displaystyle b(x,s)=0\quad {\text{ for all }}s\in S}$$ implies ${\displaystyle x=0.}$ A total subset of ${\displaystyle X}$ is defined analogously (see footnote). Thus ${\displaystyle X}$ separates points of ${\displaystyle Y}$ if and only if ${\displaystyle X}$ is a total subset of ${\displaystyle X}$, and similarly for ${\displaystyle Y}$.